$\eqalign{& v = \lambda f = \frac{5}{{100}} \times 200 = 10\frac{m}{s}  \cr & v = \sqrt {\frac{F}{\mu }}  \Rightarrow 10 = \sqrt {\frac{4}{\mu }}  \Rightarrow \mu  = 0/04\frac{{kg}}{m}0/04 = \frac{{0/006}}{L} \Rightarrow L = 1/5 \times {10^{ - 1}}m = 15cm \cr} $